Save exit Wasm FP and PC in component-to-host trampolines (#4601)

* Wasmtime: Add a pointer to `VMRuntimeLimits` in component contexts

* Save exit Wasm FP and PC in component-to-host trampolines

Fixes #4535

* Add comment about why we deref the trampoline's FP

* Update some tests to use new `vmruntime_limits_*` methods

crates/runtime/src/traphandlers/backtrace.rs

@@ -114,6 +114,7 @@ impl Backtrace {
         trap_pc_and_fp: Option<(usize, usize)>,
         mut f: impl FnMut(Frame) -> ControlFlow<()>,
     ) {
+        log::trace!("====== Capturing Backtrace ======");
         let (last_wasm_exit_pc, last_wasm_exit_fp) = match trap_pc_and_fp {
             // If we exited Wasm by catching a trap, then the Wasm-to-host
             // trampoline did not get a chance to save the last Wasm PC and FP,
@@ -137,6 +138,7 @@ impl Backtrace {
             *(*state.limits).last_wasm_entry_sp.get(),
             &mut f,
         ) {
+            log::trace!("====== Done Capturing Backtrace ======");
             return;
         }
 
@@ -151,6 +153,7 @@ impl Backtrace {
                 debug_assert_eq!(state.old_last_wasm_exit_pc, 0);
                 debug_assert_eq!(state.old_last_wasm_exit_fp, 0);
                 debug_assert_eq!(state.old_last_wasm_entry_sp, 0);
+                log::trace!("====== Done Capturing Backtrace ======");
                 return;
             }
 
@@ -160,9 +163,12 @@ impl Backtrace {
                 state.old_last_wasm_entry_sp,
                 &mut f,
             ) {
+                log::trace!("====== Done Capturing Backtrace ======");
                 return;
             }
         }
+
+        unreachable!()
     }
 
     /// Walk through a contiguous sequence of Wasm frames starting with the
@@ -245,7 +251,13 @@ impl Backtrace {
 
             pc = arch::get_next_older_pc_from_fp(fp);
 
-            let next_older_fp = arch::get_next_older_fp_from_fp(fp);
+            // We rely on this offset being zero for all supported architectures
+            // in `crates/cranelift/src/component/compiler.rs` when we set the
+            // Wasm exit FP. If this ever changes, we will need to update that
+            // code as well!
+            assert_eq!(arch::NEXT_OLDER_FP_FROM_FP_OFFSET, 0);
+
+            let next_older_fp = *(fp as *mut usize).add(arch::NEXT_OLDER_FP_FROM_FP_OFFSET);
             // Because the stack always grows down, the older FP must be greater
             // than the current FP.
             assert!(next_older_fp > fp, "{next_older_fp:#x} > {fp:#x}");

crates/runtime/src/traphandlers/backtrace/aarch64.rs

@@ -30,9 +30,9 @@ pub unsafe fn get_next_older_pc_from_fp(fp: usize) -> usize {
 
     pc
 }
-pub unsafe fn get_next_older_fp_from_fp(fp: usize) -> usize {
-    *(fp as *mut usize)
-}
+
+// And the current frame pointer points to the next older frame pointer.
+pub const NEXT_OLDER_FP_FROM_FP_OFFSET: usize = 0;
 
 pub fn reached_entry_sp(fp: usize, first_wasm_sp: usize) -> bool {
     // Calls in aarch64 push two i64s (old FP and return PC) so our entry SP is

crates/runtime/src/traphandlers/backtrace/s390x.rs

@@ -5,11 +5,9 @@ pub unsafe fn get_next_older_pc_from_fp(fp: usize) -> usize {
     *(fp as *mut usize).offset(14)
 }
 
-pub unsafe fn get_next_older_fp_from_fp(fp: usize) -> usize {
-    // The next older "FP" (backchain pointer) was saved in the slot pointed to
-    // by the current "FP".
-    *(fp as *mut usize)
-}
+// The next older "FP" (backchain pointer) was saved in the slot pointed to
+// by the current "FP".
+pub const NEXT_OLDER_FP_FROM_FP_OFFSET: usize = 0;
 
 pub fn reached_entry_sp(fp: usize, first_wasm_sp: usize) -> bool {
     // The "FP" (backchain pointer) holds the value of the stack pointer at

crates/runtime/src/traphandlers/backtrace/x86_64.rs

@@ -4,10 +4,8 @@ pub unsafe fn get_next_older_pc_from_fp(fp: usize) -> usize {
     *(fp as *mut usize).offset(1)
 }
 
-pub unsafe fn get_next_older_fp_from_fp(fp: usize) -> usize {
-    // And the current frame pointer points to the next older frame pointer.
-    *(fp as *mut usize)
-}
+// And the current frame pointer points to the next older frame pointer.
+pub const NEXT_OLDER_FP_FROM_FP_OFFSET: usize = 0;
 
 pub fn reached_entry_sp(fp: usize, first_wasm_sp: usize) -> bool {
     // When the FP is just below the SP (because we are in a function prologue